ASK encoding schemes are employed in a variety of applications. An example of a system 100 employing ASK encoding can be seen in FIG. 1. In this system 100, a transmitter 102 is able to encode a signal from signal source 101 and transmit the encoded over channel 104. The receiver 106 is then able to generate signal OUT from the encoded signal. This is normally accomplished by filtering the signal (with filter 108), demodulating (with demodulator 110), and converting the signal into a digital representation (with analog-to-digital converter (ADC) 112).
With ASK encoding, there are a variety of different schemes that can be employed, the simplest being on-off keying, and the demodulator 110 can vary in configuration based on the AKS encoding scheme used. An example of a demodulator 110 can be seen in FIG. 2. Here, a peak detector or rectifier 202 is used to convert the signal from the filter 108 into a peak detect signal PEAKDET. Typically, when there is a sine wave at the base of transistor Q1 (which corresponds to a ‘1’), capacitor C1 will be charged to the peak value (or peak voltage) of the peak detect signal PEAKDET minus one base-emitter voltage drop (from transistor Q1). Alternatively, when the sine wave is missing at the base of transistor Q1, the current source 206 discharges the capacitor C1 and drives the peak detect voltage to zero. Based on this peak detect signal PEAKDET, the comparator 204 can generate a comparison result for ADC 112 using a threshold or reference voltage REF.
However, since the sine wave input for a logic high or ‘1’ bit can be allowed to have a range of amplitude levels so as to allow for varied transmission output swings and characteristics of channel 104, demodulation can become difficult. As a result of having this range of amplitude levels, the peak voltage (and, thus, the voltage held by capacitor C1) can vary, being input signal dependent. Additionally, because of this variation in the voltage held by capacitor C1, the time to reach the reference voltage REF varies, making the duty cycle input signal dependent as shown in FIG. 3. Thus, there is a need for a method and/or apparatus to improve the duty cycle performance of the demodulator 110.
Some other examples of conventional circuits are: U.S. Pat. No. 4,947,407; U.S. Pat. No. 5,319,191; U.S. Pat. No. 7,885,359; and U.S. Patent Pre-Grant Publ. No. 2008/0252367.